1. Field of the Invention
This invention relates to a method for recovering petroleum from an underground petroleum reservoir wherein an injected surfactant containing fluid is driven by a quantity of water which has a predetermined salinity level designed to prevent partitioning of the surfactant into the oil and/or emulsion phase and thereby retaining maximum effectiveness of the surfactant.
2. Description of the Prior Art
The crude oil which has accumulated in subterranean reservoirs is recovered or produced through one or more wells drilled into the reservoir. Initial production of the crude oil is accomplished by "primary recovery" techniques wherein the natural forces present in the reservoir are utilized to produce the oil. However, upon depletion of these natural forces and the termination of primary recovery a large portion of the crude oil remains trapped within the reservoir. Also, many reservoirs lack sufficient natural forces to be produced by primary methods from the very beginning. Recognition of these facts has led to the development of many enhanced oil recovery techniques. Most of these techniques involve injection of at least one fluid into the reservoir to produce an additional amount of the crude oil therefrom. Some of the more common methods are water flooding, steam flooding, miscible flooding, CO.sub.2 flooding, polymer flooding, surfactant flooding, caustic flooding and in situ combusion.
Water flooding, involves injection of water into the subterranean oil reservoir for the purpose of displacing the crude oil from the pore spaces of the reservoir rock towards the producing wells. It is the most economical and widely used of the oil recovery methods. Nevertheless water does not displace oil with high efficiency because of the high interfacial tension between them and because of the resultant immiscible displacement of oil by water.
Surfactant flooding involves the addition of one or more surface active agents, usually referred to as "surfactants", to the water flood for the purpose of minimizing the water flooding problems mentioned above. This has been an area of active interest in the art of enhanced oil recovery methods for many years. For example, in 1941 U.S. Pat. No. 2,233,381 disclosed the use of polyglycolether as a surfactant which increases the capillary displacement efficiency of an aqueous flood. U.S. Pat. No. 3,302,713 discloses the use of petroleum sulfonates as effective surfactants in oil recovery operations. Other surfactants proposed for use in oil recovery processes include alkyl sulfates, alkylaryl sulfates, ethoxylated alkyl or aryl sulfates, alkyl sulfonates, alkylaryl sulfonates and quaternary ammonium salts.
While the above surfactants may be effective under ideal conditions, there are problems concerned with the use of each in most petroleum reservoirs. Some of the most serious problems arise from the effects of salinity extreme in the fluids which contact the surfactant within the reservoir. Such salinity extremes can cause surfactant precipitation and/or surfactant partitioning out of the aqueous phase and into the oil or emulsion phases. This results in an increase in retention of the surfactant within the reservoir matrices and fluids with a resultant loss of efficiency for the surfactant flooding process.